Terminating Access Domain Selection (T-ADS) is a procedure in an IMS-enabled telecommunications network to determine the access which should be used to route a terminating voice call to a user equipment (UE) (i.e. a call directed to the UE). In order to correctly handle the call, the SCC-AS of the IMS needs to know which access type the UE is connected via, for example the handling of the call will differ significantly between a 2G access such as GSM, and an LTE access such as E-UTRAN.
In order to obtain the radio access type (RAT) used by the UE, the SCC-AS sends a T-ADS query to the home subscriber server, HSS, of the IMS network. The HSS then queries the mobility management entity, MME, and/or SGSN to determine which RATs the UE is attached to, and a timestamp indicating the time of attachment or of the latest location update from the UE over each RAT. The RAT corresponding to the most recent update is assumed to be the RAT currently in use by the UE, and the HSS provides the identity of this RAT to the SCC-AS so that the SCC-AS can route the call appropriately. This mechanism is defined in 3GPP TS 23.292 v13.1.0, and the operation of the HSS is described in Annex E of 3GPP TS 29.328 v12.7.0.